1. Field of the Invention
This invention relates to an improved zoom lens and particularly to a high image resolution zoom lens resulting from adjusting lens groups focal length and their relative distances and by adding a filter.
2. Description of the Prior Art
A conventional zoom lens usually includes a plurality of lens groups. Through adjusting focal point distance and improving lens material, better quality images may be obtained. It is widely used in conventional film cameras and video cameras, and in recent years in digital cameras which store image in disk in digital format. The quality and resolution power of a zoom lens greatly affect the final image quality. The lens are positioned along an optimal axis. Each lens group may constitute a number of lenses of different focal length. Through adjusting the position of the lens groups, the composite focal length of the lens groups will be changed for obtaining different zooming power desired. For operational purpose, some of the lens groups will remain stationary while some other lens groups are movable. However conventional zoom lenses generally do not take total considerations of weight, size, space and resolution power of the lens groups. The difference between the first and the last lens groups usually is great. It needs special structure to provide adequate support.
U.S. Pat No. 5,285,316 discloses one of the examples. It has four lens groups 1-4 consisting of a plurality of lens L1-L11 of different focal length d1-d19. The lenses have different radius of curvature r1-r20. The focal length d1-d19 lay along an optimal axis 7. The second lens group 2 includes lenses L4-L6 to form a negative curvature and is movable along the axis 7. The optimal zooming image effect is obtained by adjusting the position of the second lens group along the optimal axis 7. The image light passes through a filter 5 and forms a final image on an imaging plane 6. There is a stop S being provided between the second and third lens group 2, 3 to prevent the second lens group 2 from hitting the third lens group 3 that might cause damage.
Because only the second lens group 2 is movable, and the focal length variation of d1-d49 is limited, the zooming power is not desirable. The resolution is also not satisfactory. Furthermore, the first lens group has a relatively larger size than the lens group 2-4. The overall lens structure needs special reinforcement to support the heavy weight of the first lens group 1. It increases production cost.